read in data, maintain decimal places

Good day, All,

Is there any way to maintain the number of decimal places in the  
type of situation below?
I would like to maintain the number of  decimal places in  0.667,  
despite the fact that its column-mates have a fourth decimal place.

Thank you for your time.

Jim

dat.txt   contents:

MARKER     ALLELES    FREQ1      RSQR      EFFECT2    STDERR      
CHISQ      PVALUE
rs6599753  C,T       .2087      .1866      0.166      0.387       
0.1850     0.6671
rs12443141 A,T       .2087      .1868      0.166      0.386       
0.1851     0.667
rs12905389 A,G       .2806      .6149      -0.038     0.193       
0.0381     0.8452

R:

dat = read.delim( dat.txt)
dat

     MARKER ALLELES  FREQ1   RSQR EFFECT2 STDERR  CHISQ PVALUE
1  rs6599753     C,T 0.2087 0.1866   0.166  0.387 0.1850 0.6671
2 rs12443141     A,T 0.2087 0.1868   0.166  0.386 0.1851 0.6670
3 rs12905389     A,G 0.2806 0.6149  -0.038  0.193 0.0381 0.8452